1. Field of the Invention
This invention relates to the manufacture of butanedicarboxylic acid esters by the cobalt catalyzed carboalkoxylation of butadiene and, more particularly, to a process for regenerating the cobalt catalyst and recycling it to the reaction without extensive additional treatment.
2. Description of the Prior Art
The cobalt catalyzed carboalkoxylation of butadiene to produce dicarboxylic acid esters as the principal reaction products has been described in the literature. U.S. Pat. No. 4,169,956, issued on Oct. 2, 1979, describes a typical stepwise carbomethoxylation of butadiene initially to methylpentenoate and then to dimethyladipate. The patent discloses preparing catalyst by reacting an aqueous cobalt salt solution with excess carbon monoxide and hydrogen in the presence of activated charcoal followed by extracting the thus prepared catalyst with butadiene. The hydrocarbon solution containing the butadiene and cobalt carbonyl hydride is then reacted with carbon monoxide and an excess of an alkanol of 1-4 carbon atoms in the presence of a tertiary nitrogen base, e.g., pyridine. A portion of the base and excess hydrocarbon is then removed from the reaction product and the resultant mixture reacted with additional carbon monoxide and alkanol at elevated pressure. The unreacted alkanol and free tertiary nitrogen base are then separated from the reactants by distillation and the product which contains catalyst, butanedicarboxylic acid ester and byproducts is treated with an oxidizing agent in an aqueous acid medium. The acidic, oxidized reaction mixture is separated into an organic phase from which butanedicarboxylic acids are eventually isolated and an aqueous phase from which cobalt salts are eventually extracted with a water immiscible solvent. The resultant phase which contains the spent cobalt (Co.sup.++) is then used to prepare fresh catalyst. The patentees further disclose that the cobalt salts obtained by extraction are advantageously treated in a strongly basic ion exchange apparently to remove any carboxylic acids present in the stream.
Another method for recovering cobalt catalyst from carboalkoxylations is described in U.S. Pat. No. 4,350,668, issued on Sept. 21, 1982 and involves the extraction of cobalt components from the reaction product with sufficient aqueous acid solution to react with all of the cobalt present. One preferred method for recovering the cobalt components from the extract involves neutralization of the extract with an alkaline solution to form cobalt hydroxide or basic cobalt carbonate which is precipitated from the solution. The cobalt salts are then recovered from the solids. The patentees also note that the reaction of acids with the tertiary amines must be avoided and for this reason their aqueous extraction process cannot be applied to the hydroesterification of butadiene in the presence of such amines.
A method of purifying carboxylic acid esters produced by reacting olefins with carbon monoxide and alkanols in the presence of a cobalt and pyridine-type catalyst is disclosed in U.S. Pat. No. 4,421,692, issued on Dec. 20, 1983, and involves the hydrogenation of the reaction mixture followed by treatment of the hydrogenate in an acid ion exchange to remove nitrogen compounds therefrom. The patentees teach that the sequence of treatment steps are critical and that the nitrogen contaminants are not sufficiently removed if the acid products are ion exchanged prior to hydrogenation.
The recycle of the reaction products from the preparation of alkylpentenoates is disclosed in U.S. Pat. No. 4,256,909, issued on Mar. 17, 1981. Variations in the reaction conditions employed for the carboalkoxylation are disclosed in U.S. Pat. Nos. 4,171,451, issued on Oct. 16, 1979; 4,310,686, issued on Jan. 12, 1982 and 4,258,203, issued on Mar. 24, 1981.